Recently moved house and found we have a very noisy/clanking boiler. When the boiler has been on for about 20 minutes it starts for a few minutes then cuts out and starts up again. This is a typical example when the boiler is on. Air in the system you may say, but I have bled the radiators about 30 times with little or no air coming out now. We have been here 2 months and drained the system twice because of a previous problem and a radiator relocation.
This is kettling I reckon. Do you have TRVs fitted to all your radiators? If so leave one open and see what happens. Are the radiators getting hot?
Generally with TRVs if they are fitted to all radiators then the boiler heats up, the TRVs close when the room is warm enough and the boiler can't dissipate the heat inside so it boils. You should have one radiator open all the time, ideally the one in the same room as your room stat if you have one.
We have 11 rads in total, 4 have TRV's (bedrooms). All radiators are getting hot. It does actually sound as though the water is bubbling (red hot). Any chance the thermotat could have gone in the boiler?
I had similar trouble with a boiler a few years ago which was caused by sludge and scale build up. As I understand it this can clog up the heat exchanger meaning that heat from the burners is not dissipated quickly enough causing the boiler to overheat and clank. I solved the problem with a couple of doses of a pretty caustic product from Fernox (DS40 as I recall) and a lot of flushing. Search the archives on Google, there are several threads on this.
However, my clanking started up pretty much as soon as the boiler came on. I can't explain why yours should take 20 minutes to start.
I have just had the plumber round and he thought he sorted it. All the time he was here nothing happened. Typical. He said it was air in the system. He bleed the pump and lots of air came out. It didn't make a noise and didn't cut out. Until now. It's started again, back to square one :O(
Hmmm. Either you have a leak which is causing air to get into the system or it could be that your stat is cream crackered as you said earlier. Does the problem go away if you turn the boiler stat down? If there's air getting in where is it coming from? Are there any tell tale leaks or patches of rust on the radiators?
isn't supposed to work as the wall stat and the water stat control the boiler. The problem seems to get a little quieter when you turn the boiler down, but this doesn't ring true with what the plumber is saying.
"I am not a plumber", but I thought the boiler *had* to have a thermostat in order to operate properly, so that the circulating water doesn't get too hot.
My own boiler is forever turning itself off (every couple of minutes after the first 15 minutes) before the CH is up to heat, because its thermostat is telling it to cool down a bit: opposite problem to yours.
Sounds to me like you have the classic CH problem: too many permutations of the components involved for an easy diagnosis to be made. And one of those components is the most expensive: the plumber/heating engineer.
I would just *love* to meet a plumber who says "Sorry mate: I can't figure this one: try another plumber!"
(As you can tell: I, also, have experienced exactly what you have. It was only cured a couple of years later when we got in a different plumber, who actually said / alleged that the previous bloke had coupled up the boiler the wrong way round. Whatever: after the second one had finished, we didn't have the same problem any more.)
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